The present invention relates to a cutting tool insert of a carbonitride alloy with titanium as main component and containing tungsten and cobalt useful for machining, e.g., turning, milling and drilling, of metal and alloys. The insert is provided with at least one wear resistant layer free from cooling cracks, which in combination with a moderate compressive stress, gives the tool insert improved properties compared to prior art tools in several cutting tool applications.
WC-Co based alloys (cemented carbide) coated with one or more layers of a wear resistant material, e.g., TiC, Ti(C,N), TiN and Al.sub.2 O.sub.3, are the dominating type of materials used for cutting tool inserts. The coatings are most often produced by employing chemical vapor deposition (CVD) techniques at relatively high deposition temperatures (700-1100.degree. C.). One weakness of such CVD-coatings in combination with WC-Co alloys is that a network of cooling cracks are formed in the coating during cooling down the CVD-load after the coating run. The cracks are caused by the mismatch in thermal expansion between the WC-Co based alloy and the coating materials. The WC-Co alloy has a thermal expansion coefficient, .alpha., in the approximate range 4.6-6.7.multidot.10.sup.-6 .degree. C..sup.-1, while typical values for the coating materials are .alpha..sub.TiC .apprxeq.7.6, .alpha..sub.TiN .apprxeq.8.0, .alpha..sub.Ti(C,N) .apprxeq.7.8 and .alpha..sub..alpha.-Al.sbsb.2.sub.O.sbsb.3 .apprxeq.7.8.multidot.10.sup.-6 .degree. C..sup.-1. This means in all cases that the coating will contract more than the WC-Co alloy upon cooling to room temperature. This contraction leads to tensile stresses in the coating which in part are relaxed by the formation of the cooling cracks.
Cooling cracks may be detrimental to the performance of the cutting tool in certain machining applications for at least three reasons:
1. The cracks act as initiation sites both for comb cracks (cracks perpendicular to the cutting edge) and edge fracture. PA0 2. The alloy, which generally is thermodynamically and chemically less stable than the coating, is exposed through the cracks to attack by cutting fluids, work piece material and the surrounding atmosphere. PA0 3. Work piece material can be pressed into the cracks during the cutting operation, thus enlarging the initial cracks.
In addition, the residual tensile stresses in the coating may lead to spalling of the coating when used in a cutting operation.
CVD-coatings on inserts of WC-Co alloys result in a reduction in transverse rupture strength (TRS) of the cutting insert which negatively influences the toughness properties of the insert. It is thought that cooling cracks and tensile stresses in the coating are of importance for this reduction.
The problem of crack formation can to a certain extent be solved by employing low temperature coating processes such as physical vapour deposition (PVD), plasma assisted CVD or similar techniques. However, coatings produced by these techniques generally have inferior wear properties, lower adhesion and lower cohesiveness. Furthermore, although these techniques may be used to deposit TiC, Ti(C,N) or TiN coatings, so far it is not possible to deposit high quality Al.sub.2 O.sub.3 -coatings with good crystallinity. In the Swedish patent application 9304283-6 a method of producing essentially crack free coatings is disclosed. However, these coatings always have a specific 114-textured .alpha.-Al.sub.2 O.sub.3 layer with a certain grain size and grain shape (platelet type grains). These coatings on ordinary WC-Co alloys always possess tensile stresses.
It is generally known that a tensile residual stress in a coating can be reduced by a mechanical treatment of the coating, e.g., by shoot peening the coating with small steel balls or similar particles. The tensile stresses are released by inducing defects in the coating or by generating further cracks (see U.S. Pat. No. 123,934). Additional cracks are not desirable for conditions mentioned above and the positive effect of the induced defects will in many cases be lost during the cutting operation when the tool insert tip may reach very high temperatures (up to 1000.degree. C.).
In U.S. Pat. No. 5,395,680 a method to obtain compressive stresses in a CVD-coating is disclosed. Onto a CVD-coating a second layer is deposited by the PVD-technique. The ion bombardment during the PVD-step induces compressive stresses in the coating. The drawback of such a process is, one, that it is an expensive two-step process and, second, it is very likely that the compressive stress state will be lost as soon as the PVD-layer is worn through.
Titanium-based carbonitride alloys, so-called cermets, are today well established as tool insert material in the metal cutting industry and they are predominantly used for finishing cutting operations. The alloys consist of carbonitride hard constituents embedded in 3-25 wt-% binder phase based on Co and/or Ni. In addition to Ti, group VIa elements, normally Mo and/or W and sometimes Cr, are added to facilitate wetting between binder and hard constituents and to strengthen the binder by means of solution hardening. Group IVa and/or Va elements, i.e., Zr, Hf, V, Nb and Ta, may also be added, mainly in order to improve the thermo-mechanical behavior of the material, e.g., its resistance against plastic deformation and thermal cracking (comb cracks). All these additional elements are usually added as carbides, nitrides and/or carbonitrides. The grain size of the hard constituents is usually &lt;2 .mu.m. The binder phase normally consists of mainly cobalt and/or nickel. The amount of binder phase is generally 3-25 wt%. Furthermore, other elements are sometimes used, e.g., aluminium, which are said to harden the binder phase and/or improve the wetting between hard constituents and binder phase.
Sintered cermets generally have a highly complex microstructure with a chemically heterogeneous hard phase far from thermodynamic equilibrium. The carbonitride grains typically have a characteristic core/rim structure where the cores may be remnants of the raw material powder and/or formed during sintering. The rims are formed both during solid state and liquid state sintering. Generally, several types of cores may be found within the same alloy. The rims most often have a large gradient in chemical composition, at least in the radial direction. The chemical composition and relative abundance of both cores and rims may be varied within large limits by proper choices of raw material powder (e.g., prealloyed powders) and processing conditions. This is true even if the macroscopic chemical composition is kept constant. These variations give rise to significant differences in the physical properties of the alloys and of course also in their performance as cutting tools.
Cermets are harder and chemically more stable than WC-Co based hard materials, but unfortunately also considerably more brittle. Due to this brittleness, they lack the reliability necessary to increase their area of application to any large degree towards more toughness demanding operations. Since CVD-coatings generally increase the brittleness of the material, CVD coated cermets have not been available on the market, most probably because coatings applied by this technique have been thought to further decrease their reliability. Instead, PVD-coated cermets have been used for certain applications demanding higher wear resistance than the alloy itself.
However, CVD-coated cermets are not unknown. Patents and patent applications published so far may be divided into two categories, those concerned with modifications of the alloy composition and those focusing on adhesion of the coating. When examining the former category one finds that the alloys described have invariably been modified in ways making them distinctly different from conventional cermets. For example, in U.S. Pat. No. 5,376,466 a CVD-coated carbonitride based material is described which allegedly has superior thermoplastic deformation resistance. In order to accomplish this, the amount of binder phase has been decreased considerably (0.2-3 wt%) compared to a conventional cermet (3-25 wt%) and an additional hard phase (5-30 wt% of zirconia or stabilized zirconia) has been added. Both the low binder content and the third phase makes this material very different from a conventional cermet.
In EP-A-0 492 059, a CVD-coated cermet is described which is claimed to have both superior wear resistance and fracture resistance. This has been accomplished by a complicated sintering process which gives rise to an increased hardness in the near surface zone of the alloy, accompanied by a tungsten enrichment and binder depletion in the same zone. Again, this makes the alloy distinctly different from a conventional cermet and also has the major disadvantage that the alloy cannot be ground to any large degree after sintering since this would remove the surface zone. Grinding the alloy after sintering is often desirable, in particular for milling inserts, in order to obtain a preferred final shape and size.
EP-A-0 440 157 and EP-A-0 643 152 fall into the second category of patents and patent applications. In these applications, different methods are described that produce sufficient adhesion between coatings and conventional cermets so that the superior wear resistance of the CVD-coating material can be utilised. In particular, it is claimed that a thin TiN or Ti(C,N) layer applied as a first coating layer onto the alloy acts as a sufficiently effective diffusion barrier for binder metal atoms to avoid, that these atoms interfere with the growth of subsequent layers.
The basis of the present invention is to combine essentially conventional CVD-coatings and conventional cermets in such a way that a dramatic increase in toughness is obtained.